(NOT APPLICABLE)
(NOT APPLICABLE)
The present invention relates generally to industrial vehicles such as boom lifts, platform vehicles and the like and, more particularly, to an extendible axle assembly for such vehicles incorporating four-wheel independent steering control.
An important design consideration for industrial vehicles such as boom lifts and the like relates to vehicle stability, particularly when the boom or platform is raised or extended or otherwise loaded to create an overturning moment on the vehicle. With a given overturning moment, the vehicle stability is a function of the amount of counterweight provided or the position of the ground contact points. These ground contact points, in the case of this present invention, are the wheels or tires on the vehicle chassis. For a given weight, the wider the chassis, the more stable the vehicle. Therefore, lighter more cost effective vehicles can be obtained with wider chassis designs. This is in direct conflict with the desire to minimize the chassis width to allow for ease of transport when the stability is not a concern, particularly when the vehicle must meet the requirements of highway transportation limitations for size.
To address this concern, it would be desirable to provide a vehicle incorporating extendible axles so that a chassis can be maintained narrow enough for on-highway transportation without special permits while providing a satisfactory amount of stability during use and operation of the vehicle lifting components. Typically, vehicles incorporating extendible axles use axles that share a common centerline either by telescoping one out of the other or meeting in the middle of the chassis end to end. Either design limits the maximum practical ratio of the retracted width to extended width without sacrificing structural integrity. This ratio of retracted to extended widths can be increased by offsetting the moving axles to either a side by side or an over and under design. Offsetting the axles greatly increases the difficulty in providing mechanical tie rods or other means of maintaining mechanical steering control between the wheels. It is desirable to control the steering of each wheel in an offset axle design individually as opposed to using mechanical tie rods or the like. With the elimination of the tie rod, the steering can be controlled electronically instead of mechanically providing an additional benefit not otherwise possible.
When a vehicle is propelled through a turn, the outside wheels travel a greater distance than the inside wheels. To prevent wheel skidding and power loss, provisions must be made in the drive system to allow the inside wheels to roll at a slower speed than the outside wheels. In the past, hydrostatic drive systems have incorporated means to allow the hydraulic flow to compensate for the speed of the inside and outside wheels for the tightest possible turn regardless of the actual turning angle. This is typically done with flow dividers with leakage provisions sufficient to span the difference in wheel flow requirements from the inside to outside wheels. This creates inefficiencies in the drive system that results in loss of tractive effort regardless of wheel angle. This characteristic is especially important on extending axle vehicles with large steering angle capability due to the large disparity of wheel speeds generated between the inside and outside wheels. To overcome this inherent loss of flow dividers, the steering sensors can be used to predict the rolling path and therefore the required wheel speed of each wheel as the steering angles change and steering modes change. The control system can then command the ideal flow from each of the two drive pumps, one for the right side of the machine and one for the left side.
In an exemplary embodiment of the invention, a vehicle is provided comprising a chassis supporting two front axle assemblies and two rear axle assemblies, each of the front and rear axle assemblies including an extendible and retractable axle supporting an independently steerable wheel and a steering mechanism. The respective axles of the front and rear axle assemblies are offset from each other. The vehicle is operable in a plurality of steering modes including two-wheel steer, crab steer and coordinated steer, wherein a steering angle for each steering wheel is determined according to a steering angle of an inside front wheel on a turn-by-turn basis. A wheel angle sensor may be disposed in cooperation with each wheel, and drive speed may be controlled on a proportional basis dependent on steer angle. Moreover, in the crab steering mode, all of the wheels are preferably controlled to substantially the same angle. In the coordinated steering mode, a projected vehicle pivot point is determined by an intercept of a line drawn perpendicularly through the center of the inside front wheel and a line drawn horizontally from a midpoint between the front and rear axle assemblies. In this arrangement, in the two-wheel steer mode and the coordinated steering mode, inside wheel rolling speed is slower than outside wheel rolling speed.
The vehicle may additionally include an operator-controlled steering switch communicating with at least the steering mechanisms of the front axle assemblies. In this context, the inside front wheel steering angle is determined according to a wheel position of the front inside wheel. In the two-wheel steering mode, a projected vehicle pivot point is determined by an intercept of a line drawn perpendicularly through the center of the inside front wheel and a line drawn horizontally from a midpoint between the axles of the two rear axle assemblies.
Each of the front and rear axle assemblies preferably includes a hydraulic cylinder coupled with the extendible and retractable axle, wherein fluid flow to the hydraulic cylinders is controlled with two directional valves, a first valve to extend and retract the front axle pair and a second valve to extend and retract the rear axle pair. The hydraulic cylinders may be controlled to effect extension and retraction of the axles only when the vehicle is traveling above a predetermined minimum speed. Additionally, four digital or analog switches may be coupled with the axles, respectively, the switches indicating that the axles are fully extended. In this context, drive and steering functions are controlled according to signals from the switches.
In another exemplary embodiment of the invention, a method of controlling vehicle steering and axle position with the vehicle of the present invention includes the steps of selectively operating the vehicle in one of a plurality of steeling modes including two-wheel steer, crab steer and coordinated steer; and determining a steering angle for each steering wheel according to a steering angle of an inside front wheel on a turn-by-turn basis.